The cops who unleashed a fatal, 50-bullet barrage on an unarmed Queens groom used “excessive force” that was “unacceptable,” a grim-faced Mayor Bloomberg said yesterday – even as he urged New Yorkers not to rush to judgment.

The officer who fired 31 of those shots was 12-year-veteran Detective Mike Oliver, who was photographed yesterday by The Post leaving his lawyer’s downtown office.

In his harshest criticism ever of police during his mayoralty, Bloomberg acknowledged that even though the probe into the racially charged shooting is ongoing, “It’s hard to understand – keep in mind I was not there at the time – why 50-odd shots should be taken.

“To me, that sounds excessive and unacceptable,” the mayor admitted to reporters at City Hall after an at-times tense, closed-door powwow with elected officials and black community leaders outraged over the killing.

“I can tell you that it is unacceptable or inexplicable how you can have 50-odd shots fired, but that’s up to the investigation to find out what really happened.”

Police Commissioner Ray Kelly, asked last night on NY1 if he agreed that excessive force was used, said only that the mayor is “entitled to his opinion.”

The victim, Sean Bell, 23, was set to marry his high-school sweetheart and mom of their two young daughters on Saturday.

But he was cut down in a police undercover operation gone horribly awry only hours earlier, after he and his pals left a seedy strip joint in South Jamaica.

The undercover officers involved in the shooting say they thought a man in Bell’s group had a gun when one tried to stop him in his car outside the club, sources have said. Then, after Bell began ramming his Nissan Altima into one of the plainclothes cops, the officer started firing – and set off a deadly exchange of bullets between him and confused backup detectives who thought the gunfire was coming from the victim’s car.

Oliver told investigators he hadn’t realized how many shots he had fired. He said when that when he emptied his first clip, he thought the gun had jammed after he squeezed off only a couple of bullets. So he quickly reloaded and emptied the new clip. The officer had never before fired his weapon in the line of duty. In fact, a source said, he’d been shot at years ago and did not fire back.

Queens District Attorney Richard Brown is set to impanel a grand jury to investigate the incident, which has already led hundreds to take to the streets to protest the officers’ actions and demand that Kelly resign.

Bell was black, as are his two friends who were wounded.

Oliver is of Syrian-Lebanese descent; his colleagues at the scene were two blacks and two whites.

Bloomberg said that even if Bell’s car was being used as a weapon against the cops, it was against NYPD policy to fire at it.

“The procedure is that you don’t fire at a car if the car is being used as a weapon,” he said. “Whether or not the police had reason to believe that there was a gun involved, I don’t know.”

In any event, he said, up until the point Bell struck the undercover with the Altima, “There is no evidence that they [he and his friends] were doing anything wrong.” Kelly said officers are taught not to fire more than three shots at a time without pausing to assess the situation.

“It’s something we stress all the time,” Kelly said. “This is part and parcel of our training.”

The shooting has already been compared to one of the city’s most infamous fatal police slayings: that of West African immigrant Amadou Diallo. The innocent black man was shot 19 times in his Bronx by four white cops, who thought he was reaching for a gun when he was only fumbling for his wallet. The officers were acquitted.

The five cops in the latest shooting have been placed on administrative leave – stripped of their guns – while the probe continues.

Like Oliver, the other four cops had never before been involved in a shooting, officials said.

The medical examiner said Bell was hit four times and died from wounds to the neck and torso.

The Rev. Al Sharpton, who has been at the forefront of those calling for action against the cops, briefly stood onstage with the mayor after their meeting – then left before Bloomberg spoke.

He later called the session “very candid,” adding that his contingent’s message to the mayor was:

“This city must show moral outrage that 50 shots were fired on three unarmed men.”

Many other black leaders flanked Bloomberg as he spoke, although some vowed to punish the city if justice is not served.

“We’re making sure that the actions of these police officers are addressed the way they should be,” said Edward Williams, a friend of Bell’s family and the president of the NAACP’s Far Rockaway branch.

“We don’t expect to allow this to be another Amadou Diallo case and just move the venue and just let folks walk. We don’t expect that, but if they do, they will hear from all over the city like they’ve never heard before.”

Williams particularly homed in an NYPD regulation that permits undercover officers to drink on duty if it helps them blend in.

Kelly said one undercover cop had two beers in the club and was not given a Breathalyzer because “we normally don’t do that.” Kelly added that the commander on the scene made an assessment that all the officers were fit for duty.

Harlem Rep. Charles Rangel, who attended the session with Bloomberg, hailed the mayor for reacting quickly to the crisis.

The latest shooting “reminds me of a tragedy that took place with Mr. Diallo. And we can’t have that. We can’t have that,” Rangel said.

Sharpton added, “This man [Bloomberg] has better manners than his predecessor. Let’s see if we have better policy.” But Bloomberg was quickly castigated by police-union officials who accused him of rushing to judgment.

“Only the detectives and police officer who fired their weapons can explain the circumstances under which they took action, and they have not yet been heard,” said Patrick Lynch, president of the Patrolmen’s Benevolent Association.

“Premature statements made without the benefit of all of the facts only serve to inflame tensions and prejudice the rights of those who are presumptively innocent.” Lynch’s sentiment was echoed by the union representing detectives. “In this particular situation, the amount of shots were not excessive,” said Detective Endowment Association President Michael Palladino. “They were having deadly physical force used against them.”

Palladino said the detectives in the case were so firm in their resolve that they were willing to testify before the grand jury without immunity.

Lieutenant takes cover – Sitting in front passenger seat of Camry as violence erupts. Ducks under dashboard and climbs out of car to the ground, where he crawls on his belly to front of car to take cover. Does not fire.

Detective fires 1 shot – Driving Camry. Tries to help block in Altima but passes it and screeches to halt down street. Jumps out and starts firing.

Detective fires 31 shots – 12-year veteran Mike Oliver says he thought his gun had jammed after several shots, so he changes the clip and empties another.

Undercover officer fires 11 shots – He follows victims from bar to their Altima. Thinking they have gun, tries to stop them by jumping on hood. Is rammed twice by Altima before being first to open fire.

Police officer fires 3 shots – One of two plainsclothes cops jumps out of van as Altima smashes into their vehicle and undercover officer screams, “They have a gun!” Begins shooting, thinking shattered glass from undercover’s bullets is gunfire coming from inside Altima.